The AP Rounds are an attachment featured within Call of Duty: Black Ops 4. As their name suggests, they increase damage against enemy armor, and also against equipment and vehicles. The attachment is available on most submachine guns, assault rifles, and light machine guns.
The attachment doesn't have any detractions other than the fact that it takes up an attachment slot. However, the AP Rounds by nature are extremely situational and require the player to actively seek out and destroy equipment and vehicles using a bullet weapon, whereas the availability of the Hellion Salvo in the secondary slot makes this playstyle effectively redundant. The player may find that it is more beneficial to bring along a Quickdraw, Grip, or High Caliber, as these attachments benefit the player during most engagements and not just against equipment and vehicles.